(1) Field of the Invention
The present invention generally relates to a cooling system for an internal combustion engine, and more particularly to a cooling system for cooling a cylinder by means of a coolant which flows in a passage formed on an outer surface of a cylinder liner.
(2) Description of the Related Art
Japanese Laid-Open Utility Modes Application No. 63-168242 discloses a cooling system for cooling an internal combustion engine by passing a coolant through a spiral or ring-shaped passage formed on an outer surface of a cylinder liner.
FIGS. 1A, 1B and 1C are respectively a plan view, a sectional view seen along a line Ib--Ib shown in the FIG. 1A, and a sectional view seen along a line Ic--Ic shown in the FIG. 1A of one example of a prior art cooling system. A plurality of ring-shaped coolant grooves 2 are formed on an outer surface of a cylinder liner 1. In the condition where the cylinder liner 1 is fitted in a bore part formed in a cylinder block 3, coolant passages 4 consist of the coolant grooves 2 and an inner surface 3a of the bore part of the cylinder block 3. Further, all of the coolant passages 4 are connected with each other by means of connecting passages 5, 6 extending along a longitudinal axial direction of the cylinder liner 1. Each passage 5, 6 consists of grooves formed on both the outer surface of the cylinder liner 1 and the inner surface 3a of the bore part of the cylinder block 3. Both of these connecting passages 5, 6 have the same sectional area. A supply pipe 7 is connected with a bottom end part of the connecting passage 5, and a drain pipe 8 is connected with a top part of the connecting passage 6. Both pipes 7 and 8 are formed in the cylinder block 3.
Coolant flows into the connecting passage 5 via the supply pipe 7 and is distributed into each of the coolant passages 4. The coolant flowing in the coolant passages 4 is exposed to heat from the cylinder liner 1, thus cooling the cylinder liner 1. The coolant is collected in the connecting passage 6 after passing through the coolant passages 4 and then the coolant flows out of the connecting passage 6 via the drain pipe 8.
In a construction of a coolant passage provided for the cylinder liner of a cooling system as shown in the FIGS. 1A through 1C, coolant flows in a parallel manner in each passage of the plurality of ring-shaped passages 4. Pressure loss incurred in the coolant passage of this type is smaller than in a spiral-shaped coolant passage which surrounds the circumference of the cylinder liner so that coolant flows occurs in one direction, from an inflow part to a flux part. Thus, it is possible to minimize a capacity of a discharging pump for circulating coolant through the coolant passages in the construction as shown in the FIGS. 1A through 1C.
FIG. 2 is a graph showing a relation between a position Z of each of the coolant passages 4 in the cooling system shown in the FIGS. 1A through 1C in an axial direction of the cylinder liner 1, and a flow velocity S of coolant flowing in each of the coolant passages 4 which corresponds to a rate of heat transmitted from a wall of the cylinder liner to the coolant.
A broken line A of the FIG. 2 shows a flow velocity distribution of coolant in each of the coolant passages 4 where a diameter of each of the connecting passages 5, 6 is relatively large. A solid line B of the FIG. 2 shows a flow velocity distribution of coolant in each of the coolant passages 4 where a diameter of each of the connecting passages 5, 6 is small.
In the above mentioned first case, relatively small pressure loss of coolant flowing in each of the coolant passages 4 is incurred where the diameter of each of the connecting passages 5, 6 is large. Thus, a flow velocity of coolant in each of the coolant passages 4 is uniform over all positions from a top position to a bottom position as shown in FIGS. 1B and 1C thereof, and as shown by the broken line A of the FIG. 2. On the other hand, in the above mentioned second case, a significant amount of pressure loss of coolant flowing in each of the coolant passages 4 is incurred where the diameter of the connecting passages 5, 6 is small. Thus, a flow velocity distribution of coolant in each of the coolant passages 4 is such that the coolant closer to the passages 4 at a top or bottom end of the cylinder liner as shown in FIGS. 1B and 1C, experiences an increased flow velocity. In a position near a central part of the coolant passages 4 as shown in FIGS. 1B, 1C, a flow velocity of the coolant decreases as shown in the solid line B of the FIG. 2.
Further, FIG. 3 is a graph showing a general relationship between position Z in the cylinder liner along an axial direction thereof and heat quantity Q incoming into the cylinder liner in an operation of the engine. In the graph, the above mentioned relationship is as shown by the line C of the graph. Generally speaking, greater amounts of heat are emitted at positions closer to a combustion chamber end of the cylinder. Thus, the higher positions of the cylinder liner are exposed to greater amounts of heat. Furthermore, lesser amounts of; heat quantity are emitted to positions farther from a combustion chamber end of the cylinder or at the lower position of the cylinder liner.
In a construction of the cooling system for cooling a cylinder liner by coolant circulating along a circumference of the cylinder liner, effective cooling is achieved by cooling to a certain temperature using a proper quantity of coolant. For example, a desired cooling system has a proper heat-transmission rate and a proper heat-transmission area, while allowing for a reduction in size of the engine and minimizing energy used for operation of the engine. However, in the construction of the cooling system 9 as shown in the FIGS. 1A through 1C, the coolant flow velocity S shown in FIG. 2, that is, a distribution of heat-transmission rate, does not coincide with the distribution of the incoming-heat rate Q even if the diameters of the connecting passages 5, 6 are altered. Thus, it is not possible to perform a cooling corresponding to the heat quantity emitted to the cylinder liner. Therefore, a problem may arise in that at one position along the axial length of the cylinder liner 1, a flow velocity of coolant flowing in the coolant passages 4 is so low that the coolant may boil due to insufficient cooling of the cylinder liner 1. At a another position along the axial length of the cylinder liner, a flow velocity of coolant is so high that excessive cooling may result in an increase of energy lost due to friction resulting from movement of a piston therein. Thus, it is not possible to perform cooling effectively.